Papers by Author: Zheng Yi Jiang

Abstract: Micro forming is a promising technology and has drawn global attentions due to the urgent requirements on miniaturised products. Micro hydro deep drawing (MHDD) is a typical micro forming method and its products are widely applied in various fields, such as micro electro-mechanical systems (MEMS), medical and aerospace areas. However, tiny sizes of both products and tools are obstacles to investigate this novel technology. Therefore, finite element method (FEM) is adopted and modified to study the influence of processing parameters on micro drawing process and produced products. In this study, a normal hydro deep drawing model was developed, and then a voronoi blank model was generated to consider the size effects of materials. Additionally, a surface layer model was created based on the voronoi blank model. Therefore, the open and closed lubricant theory can be fulfilled and friction behaviour in MHDD can be considered. Subsequently, different processing parameters, such as hydraulic pressure and material inhomogeneity, were investigated. Simulation results indicate that a proper hydraulic pressure improves drawability and shape accuracy of the drawn cups. Moreover, material inhomogeneity affects the quality of drawn cups.

Abstract: The aim of this study is to understand the effect of zinc dialkyl dithio phosphate (ZDDP) films on sticking defects during the hot rolling of ferritic stainless steel strips. The surface characterisation and crack propagation are very important for the sticking defects of ferritic stainless steel strip. A finite element method (FEM) model is constructed with different crack size ratios, in which the profile of the strip roughness and ZDDP films are taken into consideration. Simulation results show that the widths of cracks tend to be reduced with the introduction of ZDDP films, improving the sticking defects, which is confirmed by the hot rolling trials.

Abstract: To obtain a low cost, environmentally friendly and effective lubricant, a novel nanoTiO₂ additive oil-in-water (O/W) lubricant for hot steel rolling has been developed. Based on the contact angle, tribological and hot rolling tests, the performance of the nanoTiO₂ additive O/W lubricants was evaluated and discussed. The results show that the strongest adhesion force between the nanoTiO₂ additive O/W lubricant and the work roll is obtained after addition of 4% nanoTiO₂ additive in the 1% O/W lubricant. The nanoTiO₂ nanoparticles can reduce coefficient of friction (COF) and rolling force, and enhance the surface quality of the hot-rolled product. The lubrication mechanism of nanoTiO₂ additive O/W-based lubricant has been proposed.

Abstract: The effect of carbide orientation on the dry sliding wear behaviour of high chromium cast iron was studied by pin-on-disc type wear tests at room temperature. The carbide anisotropy was achieved by thermomechanical treatments at temperatures of 950 and 1150 °C. By cladding with low carbon steel, the brittle high chromium cast iron was hot compressed severely with crack free. The thermomechanical treatments not only change the carbide orientation, but also increase the volume fraction of carbides. Due to the long axis of carbide rods is parallel to the wear surface, the high chromium cast iron treated at 1150 °C has a superior wear resistance than the as-cast one, in which the long axis of carbides is perpendicular to the wear surface. For the high chromium cast iron treated at 950 °C, high volume fraction of carbide pits accelerates the wear rate significantly even though it has a similar carbide orientation as the sample treated at 1150 °C. The observations on wear tracks reveal that the ferrous matrix can be protected better from abrasion when the high chromium cast iron was treated at 1150 °C.

Abstract: High-speed steels are successfully manufactured to make work rolls in hot rolling steel mills. An understanding of their oxidation behaviour is crucial to the degradation of work rolls and the surface quality of rolled strip. In this paper, the high temperature oxidation behaviour of a work roll with high-speed steel material was investigated under isothermal conditions from 550 to 750 °C for 30 min in 20% humid air. The results indicate that molybdenum-rich carbides M₂C are oxidised and protrude out of the surface, vanadium-rich carbides MC and iron and chromium-rich carbides are easily identified and less influenced, but the oxides grown on the matrix are relatively even at 550 and 600 °C. However, at higher temperature, MC carbides are severely oxidised and extend to their nearby areas, whereas M₇C₃ zones can still be recognised but with more small particles grown on them.

Abstract: The effects of tungsten (W) addition on the microstructure, impact toughness and tensile properties of a microalloyed cast steel were systematically investigated. The results indicate that W alloying in microalloyed cast steel plays a positive role in inhibiting dendrite during casting process, and the yield strength, ultimate tensile strength and fracture strain can be simultaneously improved after W addition. Both microalloyed cast steels with and without W additions show very low impact energies, indicating W addition has no significant effect on the impact toughness. The present work provides a possibility to develop new W-containing microalloyed steels with improved mechanical properties.

Abstract: Cold rolled thin strip has received a great deal of attention through technological and theoretical progress in the rolling process, as well as from researchers who have focused on some essential parameters of strip such as its shape and profile. This paper describes the development of a 3-D finite element model of the shape of thin strip during cold rolling to simulate the cold rolling of WCS (work roll crossing and shifting) in asymmetric rolling. This finite element model considers the asymmetrical rolling parameters such as variations in the diameters of the rolls and the crossing angle as the work roll shifts on the strip during cold rolling. The shape and profile of the strip are discussed in the asymmetrical and symmetrical rolling conditions, while the total rolling force and distribution of stress are discussed in the case where the roll cross angle and axial shifting roll changes. The results can then be used to control the shape and profile of thin strip during rolling.

Abstract: The composition and phase transformation of oxide scale in cooling process (after hot rolling) of rolled microalloyed steels affect tribological features of rolled strip and downstream process, and the produced steel surface quality. In this study, physical simulation of surface roughness transfer during cooling process with consideration of ultra fast cooling (UFC) was carried out in Hille 100 experimental rolling mill, the obtained oxide scale was examined with SEM to show its surface and phase features. The results indicate that the surface roughness of the oxide scale increases as the final cooling (coiling) temperature increases, and the flow rate of the introduced air decreases. The cracking of the surface oxide scale can be improved when the cooling rate is 20 °C/s, the strip reduction is less than 12 %, and the thickness of oxide scale is less than 15 μm, independent of the surface roughness. A cooling rate of more than 70 °C/s can increase the formation of retained wustite and primary magnetite precipitates other than the precipitation of α-iron. This study is helpful in optimising the cooling process after hot rolling of microalloyed steels to obtain quality surface products.

Abstract: The dry sliding wear behaviour of the full pearlite in a novel bimetal consisting of low carbon steel and hypoeutectoid steel has been studied by means of pin-on-disc type wear tests at room temperature. Thermomechanical treatments were performed on the bimetallic samples to obtain different interlamellar spacings. It was found that interlamellar spacing decreased with an increase in plastic strain to a great extent initially and followed by a lower extent of decrease. This decrease not only increases the hardness and strain hardening capacity of the fully pearlitic microstructure, but also is in favor of stabilizing the friction coefficient during sliding process. The observations of wear tracks show that delamination dominated the wear process when interlamellar spacing is higher than 200 nm, while pronounced oxidational wear occurred with interlamellar spacing below 200 nm.

Abstract: The high carbon steel (HCS)/low carbon steel (LCS) laminated composite made by centrifugal casting technology was subjected to hot compression tests on Gleeble 3500 thermomechanical simulator in a range of temperatures (800-1100 ^oC) and strain rates (0.02-10 s^-1). The hot-working behavior of the laminate was characterised by analysing the flow stress-strain curves and constructing the processing map based on dynamic materials model via superimposing efficiency of power dissipation and flow instability maps. The safe and unsafe processing conditions were identified in the processing map which was validated by microstructural examinations. Banded microstructure and micro-shear cracks occurred in the unsafe domains were responsible for the flow instability, while dynamic recrystallisation in stable domains with high efficiency of power dissipation imparted a good workability to the laminate. The optimum hot-working parameters were determined to be: (i) 800-1050 ^oC and 0.02-0.04 s^-1, (ii) 800-1045 ^oC and 2.5-10 s^-1 and (iii) 1050-1100 ^oC and 0.02-2.5 s^-1.